1. Field of the Invention
The present invention relates to an AD converter, which quantizes input analog signals and converts to digital signals and outputs them, more particularly, to technical field of the delta sigma type AD converter having band pass filter transmission function.
2. Description of the Related Art
In recent years, public attention has been paid to a digital tuner, which receives analog broadcasting wave converts intermediate frequency (IF) signals obtained through a high frequency circuit to digital signals so as to digitize subsequent processing. The digital tuner utilizes a delta sigma type AD converter as a component for converting the IF signals to digital signals. Use of the delta sigma type AD converter enables quantization noise in the vicinity of the band of the IF signal frequency to be damped sufficiently, so as to achieve the quantization with a high resolution.
Generally, in the delta sigma type AD converter, the frequency of the IF signal to be input is set xc2xc the sampling frequency. For example, if 10.7 MHz, which is a general IF frequency, is employed, the sampling frequency is set to 42.8 MHz. Alternatively, if folded frequency by the sampling frequency is used, the frequency of the IF signal to be input may be set xc2xe the sampling frequency. In this case, the sampling frequency is set to 14.25 MHz to the IF signal of 10.7 MHz.
As described above, in the delta sigma type AD converter, the relationship between the frequency of the input signal and sampling frequency is fixed. However, the relationship of the frequency may be sometimes required to be changed for the reason of factors on the design of the apparatus. For example, a crystal vibrator is employed as reference of the sampling frequency and in some cases, its harmonic components overlap a receiving band of the digital tuner thereby disturbing it. To cope with such a case, it can be considered to change the sampling frequency and simultaneously change the frequency of the IF signal. However, although in the aforementioned 10.7 MHz case cheap components can be used because the frequency has be spread widely as the frequency of the IF signal, utilization of any special IF frequency raises the price of the components thereby inducing an increase of cost of the entire apparatus. In a conventional delta sigma type AD converter, the relationship of the frequency is apt to be fixed, so that it becomes difficult to adjust the relationship of the frequency depending on factors on the design.
Accordingly, the present invention has been achieved in views of the above problem problems and therefore, an object of the invention is to provide a delta sigma type AD converter in which by controlling a feedback amount from a delay device, the relationship between the IF signal and the sampling frequency can be set freely without being fixed, so that it can be adjusted corresponding to factors on the design and usage condition.
The above object of the present invention can be achieved by a n-order delta sigma type AD converter for quantizing an input analog signal and converting it to an output digital signal, provided with: a signal processing device which executes filter processing corresponding to a transmission function to said input analog signal; and a quantizing device which quantizes the input analog signal subjected to said filter processing with a sampling frequency, converts it to output digital signal, and outputs, wherein when quantizing noise applied by said quantizing device is Q(Z) and a predetermined coefficient is xcex1, said output digital signal includes a component indicated by (1+xcex1Zxe2x88x921+Zxe2x88x922)nQ(Z).
According to the present invention, the input analog signal to the delta sigma type AD converter is subjected to filter processing corresponding to a predetermined transmission function and after that, quantized with a predetermined sampling frequency and converted to output digital signal. At this time, quantization noise Q(Z) added by the quantizing device is output as the component (1+xcex1Zxe2x88x921+Zxe2x88x922)nQ(Z) based on operation of the aforementioned transmission function. Therefore, the frequency characteristic of the quantization noise Q(Z) can be adjusted freely depending upon setting of the coefficient xcex1, the relationship between the input analog signal and the sampling frequency does not have to be fixed and a frequency relationship fitting to design factors and use condition can be set up.
The above object of the present invention can be achieved by a delta sigma type AD converter for quantizing an input analog signal and converting it to an output digital signal, provided with: a computing device which subtracts a first feedback signal and a second feedback signal from said input analog signal and adds a third feedback signal thereto; a delay device which outputs the output of said computing device with a delay through two delay elements connected in series; a first feedback device which outputs the output of said delay device as said first feedback signal and multiplies the output of the delay element on a preceding stage contained in said delay device with a predetermined coefficient and outputs it as said second feedback signal; a quantizing device which quantizes the output of said delay device with a sampling frequency, converts it to output digital signal and outputs; and a second feedback device which converts said output digital signal to an analog signal and outputs it as said third feedback signal.
According to the present invention, the input analog signal to the delta sigma type AD converter is subjected to addition and subtraction relative to various feedback signals by the computing device, and its output is delayed by the delay device. After that, it is quantized by a predetermined sampling frequency and converted to output digital signals. On the other hand, a first feedback signal which is post stage output by the delay device composed of two stages and a second feedback signal which is obtained by multiplying the same output with a predetermined coefficient are output by a first feedback device. A third feedback signal which is obtained by converting the output digital signal to the analog signal is output by a second feedback device. Consequently, the frequency characteristic of quantization noise applied by the quantizing device based on setting of a predetermined coefficient can be adjusted freely by the operation of the first and the second feedback device. Thus, the relationship between the input analog signal and the sampling frequency does not have to be fixed like the first aspect of the invention, so that the frequency relationship fitting to design factors and use condition can be set up.
In one aspect of the converter of the present invention, respective stages each containing said computing device, said delay device, said first feedback device, said quantizing device, and said second feedback device are connected through n stages so as to construct n order.
According to the present invention, the delta sigma type AD converter of the n order can be realized by constructing the first stage with the components mentioned in the second aspect of the invention and connecting n stages thereof. Therefore, the n is set large if it is intended to damp the quantization noise largely and if reduction of cost is intended by simplifying the structure, the n is set small. Consequently, not only the above-described freedom in the frequency relationship is ensured, but also balance between performance and cost can be attained by selecting an optimum order.
In another aspect of the converter of the present invention, said first feedback device is constructed so as to be capable of changing over two or more coefficients as said predetermined coefficient.
According to the present invention, in addition to the structures mentioned in the second and third aspects, plural coefficients are prepared as the first feedback coefficient and they can be switched over. Therefore, by changing the quantity of the feedback from the pre-output of the delay device, the relationship between the frequency of the input analog signal and the sampling frequency can be switched and if it is used under a different use condition, an appropriate frequency relationship can be set up selectively.
According to the present invention, the relationship between the input frequency and the sampling frequency can be set freely without being fixed by controlling the amount of the feedback from the delay device in the delta sigma type AD converter. Consequently, it is possible to realize the delta sigma type AD converter which can be adjusted appropriately depending upon design factors and use condition.
In further aspect of the converter of the present invention, said input analog signal is an IF signal corresponding to broadcasting wave and said predetermined coefficient and said sampling frequency are set so that the frequency characteristic of said quantization noise has a transmission zero point in the frequency band of said IF signal.
According to the present invention, the analog IF signal is converted to digital signal by applying the aforementioned delta sigma type AD converter to a broadcasting receiver unit or the like. Therefore, by adjusting the transmission zero point of quantization noise so as to coincide with the frequency band of the IF signal, the IF signal can be quantized at a high resolution thereby improving reception performance.
In further aspect of the converter of the present invention, said predetermined coefficient and said sampling frequency are set so that the harmonics of said sampling frequency does not overlap the reception band of said broadcasting wave.
According to the present invention, in addition to the same operation as the fifth aspect, the sampling frequency can be set so that the harmonics of the sampling frequency does not overlap the reception band of broadcasting wave. Therefore, it is possible to prevent the reception performance from being deteriorated by jump into radio system of the harmonics of the sampling frequency.
In further aspect of the converter of the present invention, said first feedback device is constructed so as to be capable of switching a first coefficient corresponding to a first area and a second coefficient corresponding to a second area as said predetermined coefficient and capable of selecting a first sampling frequency corresponding to said first coefficient and a second sampling frequency corresponding to said second coefficient, and said first coefficient and said first sampling frequency are set so that harmonics of said first sampling frequency does not overlap the reception band of broadcasting wave in said first area while said second coefficient and said second sampling frequency are set so that the harmonics of the second sampling frequency does not overlap the reception band of broadcasting wave in said second area.
According to the present invention, if the delta sigma type AD converter is applied to a broadcasting receiver unit or the like and made available in two different areas, two combinations between a predetermined coefficient and sampling frequency are prepared such that they can be switched over. Consequently, it can be set so that one combination does not overlap broadcasting wave reception band of one area while the other combination does not overlap broadcasting wave reception band of the other area. Thus, only by changing setting of the same delta sigma type AD converter, it is possible to prevent reception performance from being deteriorated by jump into radio system of the harmonics of the sampling frequency in two areas each having a different reception band.